// @规则嵌套和冒泡
.component {
    width: 300px;

    @media (min-width: 768px) {
        width: 600px;

        @media (min-resolution: 192dpi) {
            background-image: url(/img/retina2x.png);
        }
    }

    @media (min-width: 1280px) {
        width: 800px;
    }
}

